External seals

For bearing arrangements where the efficiency of the seal under the given operating conditions is more important than space considerations or cost, there are several possible seal types to choose from.
The seals offered by SKF are given special attention in the following section. Many ready-to-mount external seals are available commercially. For seals that are not part of the SKF range, the information provided in the following section is to be used as a guideline only. SKF takes no responsibility for the performance of these non-SKF products. Make sure to check with the seal manufacturer before designing any seal into an application.

Non-contact seals

The simplest seal used outside the bearing is the gap-type seal, which creates a small gap between the shaft and housing (fig 1). This type of seal is adequate for grease lubricated applications that operate in dry, dust-free environments. To enhance the efficiency of this seal, one or more concentric grooves can be machined in the housing bore at the shaft exit (fig 2). The grease emerging through the gap fills the grooves and helps to prevent the entry of contaminants.
With oil lubrication and horizontal shafts, helical grooves - right-hand or left-hand depending on the direction of rotation of the shaft - can be provided in the shaft or housing bore (fig 3). These serve to return emerging oil to the bearing position. It is essential here that the direction of shaft rotation does not change.
Single or multi-stage labyrinth seals are considerably more effective than simple gap-type seals, but are more expensive to produce. They are chiefly used with grease lubrication. Their efficiency can be further improved by periodically applying a water-insoluble grease, e.g. a grease with a lithium-calcium thickener, via a duct to the labyrinth passages. The tongues of the labyrinth seal are arranged axially (fig 3) or radially (fig 4), depending on the housing, split or one-piece, mounting procedures, available space etc. The width of the axial passages of the labyrinth remains unchanged when axial displacement of the shaft occurs in operation and can thus be very narrow. If angular misalignment of the shaft with respect to the housing can occur, labyrinths with inclined passages are used (fig 5).
Effective and inexpensive labyrinth seals can be made using commercially available products, e.g. using SKF sealing washers (fig 6).
Sealing efficiency increases with the number of washer sets used, or can be further enhanced by incorporating flocked washers. Additional information on these sealing washers can be found in the catalogue Industrial shaft seals.
Rotating discs (fig 7) are often fitted to the shaft to improve the sealing action of shields, and flinger rings, grooves or discs are used for the same purpose with oil lubrication. The oil from the flinger is collected in a channel in the housing and returned to the inside of the housing through suitable ducts (fig 8).

Contact seals

Radial shaft seals are contact seals that are used, above all, for sealing oil-lubricated bearing arrangements.. These ready-to-mount elastomer sealing components normally have a metal reinforcement or casing. The sealing lip is usually a synthetic rubber and is normally pressed against a counterface on the shaft by a garter spring. Depending on the seal material and medium to be retained and/or excluded, radial shaft seals can be used at temperatures between –60 and +190 °C.
The contact area between the sealing lip and counterface is of vital importance to sealing efficiency. The surface hardness of the counterface should normally be at least 55 HRC and the hardened depth should be at least 0,3 mm, the surface roughness to ISO 4288:1996 should be within the guidelines of Ra = 0,2 to 0,8 μm. In applications, where speeds are low, lubrication is good and contamination is minimal, a lower hardness can be acceptable. To avoid the pumping action produced by helical grinding marks, plunge grinding is recommended.
If the main purpose of the radial shaft seal is to prevent lubricant from leaving the housing, the seal should be mounted with the lip facing inwards (fig 9). If the main purpose is to exclude contaminants, the lip should face outwards, away from the bearing (fig 10).
V-ring seals (fig 11) can be used both with oil and with grease lubrication. The elastic rubber ring (body) of the seal firmly grips the shaft and rotates with it, while the sealing lip exerts a light axial pressure on the stationary component, e.g. the housing. Depending on the material, V-rings can be used at operating temperatures between –40 and +150 °C. They are simple to install and at low speeds permit relatively large angular misalignments of the shaft. A surface roughness Ra of between 2 and 3 μm is sufficient for the counterface. At peripheral speeds above 8 m/s the V-ring must be axially located on the shaft. At speeds above 12 m/s the ring must be prevented from "lifting" from the shaft by, for example, a sheet metal support ring. When the peripheral speed exceeds 15 m/s the sealing lip will lift from the counterface so that the V-ring becomes a gap-type seal. The good sealing action of the V-ring depends mainly on the fact that the ring body acts as a flinger, repelling dirt and fluids. Therefore, with grease lubrication the seal is generally arranged outside the housing, whereas for oil lubrication it is normally arranged inside the housing with the lip pointing away from the bearing position. Used as a secondary seal, V-rings protect the primary seal from excessive contaminants and moisture.
Axial clamp seals (fig 12) are used as secondary seals for large diameter shafts in applications where protection is required for the primary seal. They are clamped in position on a non-rotating component and seal axially against a rotating counterface. For this type of seal, it is sufficient if the counterface is fine turned and has a surface roughness Ra of 2,5 μm.
Mechanical seals (fig 13) are used to seal grease or oil lubricated bearing positions where speeds are relatively low and operating conditions difficult and arduous. They consist of two sliding steel rings with finely finished sealing surfaces and two plastic cup springs (Belleville washers), which position the sliding rings in the housing bore and provide the necessary preload force to the sealing surfaces. There are no special demands on the mating surfaces in the housing bore.
Felt seals (fig 14) are generally used with grease lubrication. They are simple and inexpensive and can be used at peripheral speeds of up to 4 m/s and at operating temperatures up to +100 ºC. The counterface should be ground to a surface roughness Ra = 3,2 μm. The efficiency of a felt seal can be much improved by mounting a simple labyrinth seal as a secondary seal. Before being inserted in the housing groove, the felt rings or strips should be soaked in oil at about 80 °C.
Spring washers (fig 15) provide simple, inexpensive and space-saving seals for grease lubricated rigid bearings, particularly deep groove ball bearings. The washers are clamped against either the outer ring or the inner ring and exert a resilient pressure axially against the other ring. After a certain running-in period these seals become non-contact seals by forming a very narrow gap-type seal.
More detailed information on seals supplied by SKF can be found in the catalogue Industrial shaft seals and seals incorporated in SKF products are also described in detail in literature dealing with these products.
More detailed information on seals supplied by SKF can be found in the catalogue Power transmission seals and seals incorporated in SKF products are also described in detail in literature dealing with these products.
SKF logo